1. Field of the Invention
The present invention relates to a harmonic measuring method in use with an electric power system, and a current injection device used for the harmonic measuring method.
2. Description of the Related Art
In an electric power system, it is very important to reduce the higher harmonics of an electric power.
The frequencies of the higher harmonics are each an integral multiple of the frequency f.sub.s of a fundamental wave of the electric power. The frequency of the fifth harmonics typically used, is 5.times.f.sub.s.
In an ordinary method for reducing the harmonic components in the electric power, the voltage levels of the harmonics are estimated and a filter for suppressing the harmonics is connected to the power line in the electric power system.
To estimate the harmonic voltage levels, a node where a filter is connected to the power line is used as a harmonic measuring point. The harmonic characteristics at points upstream and downstream from the harmonic measuring point are obtained, and the equivalent circuit is depicted on the basis of the harmonic characteristics.
The equivalent circuit may be depicted as a parallel circuit of an admittance and a current source as taught by Norton's theorem.
In connection with this, a unique approach is described in the Institute of Electrical Engineers of Japan, Vol. 101-8B, pp. 451-458 (Showa 56-8). To set up an equivalent circuit of the distribution line for the 5th harmonic, the approach measures the voltage and current of the fundamental wave of the electric power transmitted in the electric power system, and the values of the admittance and the current source, its phase and other factors of the equivalent circuit are estimated by a mathematical process.
In the harmonic measuring method described in the above paper, to obtain the equivalent circuit for the n-th harmonic (whose frequency is n.times.f.sub.s) of the electric power transmitted in the electric power system, the values of the admittance and the current source, and the phases of the equivalent circuit are determined through the estimation thereof based on the voltage and current of the electric power, which are measured. Therefore, the obtained circuit constants of the equivalent circuit are not exact.
In the conventional harmonic measuring method, to obtain the values of the admittance and current source of the equivalent circuit for the targeted harmonic as an object to be measured, the fundamental wave is measured, and a level (vector value) of the targeted harmonic is estimated on the level (vector value) of the fundamental wave. In other words, the circuit constants of the equivalent circuit is obtained not through a direct measurement of them. Therefore, the obtained equivalent circuit is unsatisfactory in its accuracy.
Thus, the harmonic characteristic in the electric power system that is obtained by the conventional harmonic measuring method is unsatisfactory in its accuracy. This makes it impossible to select a proper filter for suppressing or removing the harmonic components in or from the electric power system, and hence to effectively suppress or remove the harmonic components.
An inventive and unique technique to obtain the harmonic equivalent circuit was disclosed in Japanese Patent Application No. Hei-8-310192, filed by the applicant of the present application. In our technique, the harmonics of different frequencies are used. The frequency of a targeted harmonic is the intermediate between the frequencies of the interharmonics. The frequencies of the interharmonics are each a nonintegral multiple of the frequency of the fundamental wave of the electric power in the electric power system. The currents of the interharmonics are injected into a harmonic injection point on the power line in the electric power system. The voltage values of the interharmonics are measured at the harmonic injection point, and the current values of the interharmonics flowing through the power lines upstream and downstream from the harmonic injection point are measured. The admittance values of equivalent circuits of the power lines, located upstream and downstream from the harmonic injection point, for the interharmonics of the frequencies, are determined by use of the actual measurement results on the injected interharmonics, and those equivalent circuits are specified by the obtained admittance values.
It is noted that the interharmonics to be injected are not present originally in the electric power system, and that the admittance values of the equivalent circuits for the injected interharmonics are obtained through an actual measurement. Therefore, the obtained harmonic characteristic for the targeted harmonic is reliable.
In our harmonic measuring method, the harmonic injection point and the harmonic measuring point are set at the same point. To measure the harmonic characteristic of an upstream power line including an upstream bus, it is necessary to set the harmonic injection point on the upstream bus of high voltage.
Therefore, the harmonic injection point is at high voltage, and the current injection device is coupled with the high voltage injection point. Such a device must have a high voltage rank (insulation rank) and be satisfactorily protected against the high voltage.
The current injection device having the high voltage rank and being highly protected against high voltage becomes necessarily large in size, complicated in construction, and high in cost to manufacture. Therefore, it is impossible to simplify the device construction, to decrease the device cost, and to accurately measure the harmonic characteristic of the upstream bus.
A possible measure to cope with this is to locate the harmonic injection point at a point downstream from the harmonic injection point. In this case, one or more number of substation transformers must be located between the harmonic measuring point and the harmonic injection point. Further, the currents of the interharmonics to be injected to the harmonic injection point must be large in order to secure a reliable measurement at the harmonic measuring point. This necessitates the use of the harmonic current injecting device of an extremely large current capacity. In this case, large current injected is concentrated at one point to cause the problem of a called harmonic interference.
In our harmonic measuring method, the harmonic injection point and the harmonic measuring point are set at the same point. Accordingly, the harmonic measuring method measures the harmonic characteristic of only the whole upstream or downstream power line. In other words, the harmonic measuring method cannot individually measure the harmonic characteristics of a plural number of power lines branched downstream from the harmonic injection point.
Further, our measuring method requires the current injection device for injecting the currents of the interharmonics of the frequencies f1 and f2, which are located on both sides of the frequency of the targeted harmonic in frequency spectrum.
In case where the electric power system is of the distribution-line carrier communication type, a master station located upstream of the electric power system sends switch control information to slave stations attached to segment switches via distribution lines.
Where the harmonic characteristics of the distribution system, which is of the distribution-line carrier communication type and has the switch control function, are measured by our harmonic measuring method, if the master station and the slave stations are equipped with the current injection devices and the like, an installation cost is increased.
In this case, it is necessary to select the frequencies f1 and f2 of the injected interharmonics to be different from the communication frequency for the switch control so as to avoid the interference of the current injection for the harmonic measurement with the switch control.
Since our harmonic measuring method requires the harmonic injection device, the measuring device is inevitably increased in size. Therefore, the technical subject presented to us is how to form and inject the currents of the interharmonics with the small current injection device,